1. Field of the Invention
The present invention relates to a transmitting device, to a receiving device and to a communication method for transmitting and receiving signals modulated on frequency subcarriers of an OFDM (Orthogonal Frequency Division Multiplexing) communication system.
2. Description of the Related Art
In bi-directional burst-oriented digital communication systems there is the need to synchronize a receiving device to a data burst coming from a transmitting device. This is true for wired as well as wireless communication systems. Generally, in order to perform signal detection, AGC (Automatic Gain Control) adjustment, frequency offset estimation, timing synchronization and channel equalization on the receiving side, preambles are used. A preamble is inserted either just before the payload data part of a burst or in the middle of a burst by a transmitting device and is processed on the receiving side in order to perform the mentioned functions.
In current burst oriented digital communication systems, in which a preamble is inserted just before the payload data part, the preamble can roughly be divided in three sections. The first section comprises information or symbols which are used on the receiving side for signal detection and automatic gain control as well as a coarse frequency and timing estimation. The second section comprises symbols and information used on the receiving side for channel estimation, fine frequency offset estimation and timing synchronization. The third section comprises information and symbols relating to additional signaling information such as the used modulation scheme, the packet length, service information and so forth. In OFDM communication systems, in which data or symbols are modulated on frequency subcarriers, the first section usually comprises some kind of auto-correlation pattern, i.e. pilot symbols which are modulated onto the frequency subcarriers, for example, sequences like CAZAC (Constant Amplitude Zero Auto-Correlation) are used. Hereby, the sequences with the pilot symbols are spread over the frequency subcarriers of an OFDM symbol, whereby all carriers in between the two modulated carriers with the pilot symbols are set to zero. The idea of these special pilot patterns is to get suited correlation patterns with high correlation maximums in order to achieve a first and coarse synchronization.
The automatic gain control information is usually placed before the auto-correlation pattern. In the second section, known OFDM symbols, i.e. training sequences or pilot patterns which are known to the receiver, are mapped onto the frequency subcarriers which allow a channel equalization as well as a fine frequency and timing synchronization. The third section comprising the additional signaling data, the specific content of which basically depends on the MAC (Media Access Control) architecture.
The general problem with preambles is that with increasing preamble length, i.e. increasing length of the various training and correlation patterns, the synchronization and channel estimation probability and liability gets better, on the other hand, the overall data throughput decreases since the ratio between the transmitted preamble data and the transmitted payload data is getting larger. Particularly, in a communication network with many active nodes and therefore a high number of data exchanges with many preambles, long preambles cause a significant decrease of the effective data rate.
The object of the present invention is therefore to provide a transmitting device, a receiving device and a communication method for transmitting and receiving signals modulated on frequency subcarriers of an orthogonal frequency division multiplexing communication system, whereby bursts comprising a preamble part and a payload part are transmitted, in which the preamble part is as short as possible while still guaranteeing the secure synchronization on the receiving side.
The above object is achieved by a transmitting device for transmitting signals modulated on frequency subcarriers of an orthogonal frequency division multiplexing (OFDM) communication system according to claim 1. The transmitting device according to the present invention is adapted to transmit OFDM bursts comprising a preamble part and a payload data part and comprises preamble generating means adapted to generate said preamble part with a section consisting of pilot symbols mapped onto every n-th frequency subcarrier and signaling data mapped onto the frequency subcarriers between frequency subcarriers with the pilot symbols. Hereby, n=1, 2, 3, 4, 5, 6, . . . .
Advantageously, the pilot symbols form a cross-correlation pattern. In other words, the pilot symbols are known to the receiving side. Further advantageously, the preamble generating means modulates the signaling data of the mentioned section of the preamble part with a robust modulation scheme. Hereby, the robust modulation scheme is a modulation scheme in which symbols to be transmitted or mapped onto 2 or 4 constellation points of the modulation scheme. For example, the robust modulation scheme may be a BPSK or a QPSK modulation scheme. Alternatively, higher constellations with e.g. 8, 16, 32 or more constellation points can be used if the channel characteristics are good enough. Further advantageously, the preamble generating means provides the signaling data with a forward error correction in order to increase the probability of a correct decision at the decoder on the receiving side. Advantageously, the transmitting device of the present invention and its elements are adapted to transmit signals in a powerline communication system.
An example for such a forward error correction is the Viterbi code.
The above object is further achieved by a receiving device for receiving signals modulated on frequency subcarriers of an OFDM communication system according to claim 7. The receiving device according to the present invention is adapted to receive OFDM bursts comprising a preamble part and a payload data part, said preamble part comprising a section with pilot symbols mapped onto every n-th frequency subcarrier (n=1, 2, 3, 4, 5, 6, . . . ) and signaling data mapped onto the frequency subcarriers between the frequency subcarriers with the pilot symbols, and comprises channel estimation means adapted to perform a first channel estimation on the basis of said received pilot symbols, the result of which is used to reconstruct the entire section of the received preamble as a training pattern for an accurate channel estimation and first channel equalization means adapted to use the result of the accurate channel estimation for a channel equalization of the received payload part.
Advantageously, the channel estimation means is adapted to perform said accurate channel estimation by comparing the training pattern with the received section of the preamble part. Further advantageously, the first channel estimation result provided by the channel estimation means on the basis of the received pilot symbols is supplied to a second channel equalization means adapted to perform a channel equalization on the received signaling data. Hereby, the equalized signaling data is advantageously used to reconstruct the entire section of the received preamble as said training pattern for said accurate channel estimation.
Advantageously, the receiving device of the present invention and its elements are adapted to receive signals in a powerline communication system.
The above object is further achieved by a communication method for transmitting and receiving signals modulated on frequency subcarriers of an OFDM communication system according to claim 11. In the communication method of the present invention, OFDM bursts comprising a preamble part and a payload data part are transmitted, said preamble part having a section consisting of pilot symbols mapped onto every n-th frequency subcarrier (n=1, 2, 3, 4, 5, 6, . . . ) and signaling data mapped onto the frequency subcarriers between the frequency subcarriers with the pilot symbols, and whereby a first channel estimation on the basis of the received pilot symbols are performed, the result of which is used to reconstruct the entire section of the received preamble as a training pattern for an accurate channel estimation, which is used for a channel equalization of the received payload part.
Advantageously, the accurate channel estimation is performed by comparing the training pattern with the received section of the preamble part. Further advantageously, the first channel estimation result is used to form a channel equalization on said received signaling data. Hereby, the equalized signaling data is advantageously used to reconstruct the entire section of the received preamble as the training pattern for said accurate channel estimation. The term ‘signals’ is meant to encompass all kinds of payload data, symbols, information, training patterns, pilot symbols etc. which can be transmitted and received in a wireless or wired communication system.
The present invention bases on the idea to minimize the number of preamble symbols by merging parts of all of the signaling data from the third section of the known preambles into the channel estimation pattern of the second section of the preamble. This is achieved by replacing the training data or pilot symbols in the specific frequency plots (frequency subcarriers) of each of the training symbols with signaling data, preferably in a robust constellation pattern, then deciding the transmitted data pattern after a coarse channel estimation derived from the remaining training pattern and finally re-using the decided data pattern to reconstruct the full training pattern or symbol over all the frequency subcarriers of the section of the preamble. Combining the channel equalization and signaling information in the same section of the preamble significantly shortens the overall preamble length. It is to be noted that the length of the preamble can be made shorter the more stable or static the transmission in a communication channel is. The synchronization in static or quasi-static channels is much easier than in fast-changing transmission channels since the attenuation and the phase shift remain stable over a long period of time so that less synchronization information is necessary. Additionally, a static or quasi-static channel opens possibilities to shorten the preamble since the automatic gain adjustment is much easier than in fast-changing channels. Generally, the present invention is advantageous for static or quasi-static communication channels, as in most wired communication systems, specifically power line communication systems, DSL communication systems and so forth. For example, in a power line communication system, the transmission channels are quasi-static, since except in events like the switching of lights and the plugging or unplugging of devices the channel characteristics remain stable. The same is true for communication systems using the wired telephone network. On the other hand, also wireless communication systems may have quasi-static channel characteristics, such as e.g. WLAN communication systems. The present invention is therefore not restricted to wired communication systems, but can also be applied to wireless communication networks.